1. Field of the Invention
The present invention relates to variable displacement compressors that are used in vehicle air conditioners. More particularly, the present invention relates to a variable displacement compressor that changes its displacement by adjusting the inclination of a cam plate.
2. Description of the Related Art
Variable displacement compressors typically have a cam plate that is tiltably supported on a drive shaft. The inclination of the cam plate is controlled based on the difference between the pressure in a crank chamber and the pressure in cylinder bores. The stroke of each piston is varied by the inclination of the cam plate.
Variable displacement compressors often have a drive shaft directly connected to an external drive source such as an engine without a clutch located in between. In this clutchless system, the compressor continues to operate even when refrigeration is unnecessary or when frost is being formed in the evaporator. Japanese Unexamined Patent Publications No. 3-37378 and 7-127566 disclose variable displacement compressors that stop the circulation of refrigerant gas when refrigeration is unnecessary or when frost is being formed in the evaporator.
In a compressor according to Japanese Unexamined Patent Publication No 3-37378, introduction of refrigerant gas from an external refrigerant circuit into a suction chamber is stopped by an electromagnetic valve, thereby stopping the gas circulation. The electromagnetic valve, however, opens or shuts the passage between the external refrigerant circuit and the suction chamber too quickly. This suddenly increases or decreases the amount of gas entering the cylinder bores from the suction chamber. Sudden changes in the amount of gas flow into the cylinder bores results in abrupt fluctuation of the compressor's displacement. Accordingly, the compressor's discharge pressure fluctuates. This significantly varies the load torque of the compressor, that is, the torque necessary for operating the compressor, in a short time.
A compressor according to Japanese Unexamined Patent Publication No. 7-127566 has a valve located in a discharge passage that connects the discharge chamber and an external refrigerant circuit. When the difference between the pressure in the discharge chamber (discharge pressure) and the pressure in the suction pressure area (suction pressure) is equal to or below a predetermined level, the valve closes the discharge passage to stop refrigerant gas flow from the compressor to the external circuit. The difference between the discharge pressure and the suction pressure changes slowly. Accordingly, the valve slowly changes the cross-sectional area of the passage, through which refrigerant gas is discharged from the discharge chamber to the external refrigerant circuit, in accordance with the difference between the discharge pressure and the suction pressure. This results in mild fluctuations of the gas flow amount from the discharge chamber to the external circuit. Sudden changes in the compressor's load torque are thus prevented.
The above described valve includes a cylindrical valve body. The valve body has a face for receiving the discharge pressure and another face for receiving the suction pressure. The suction pressure receiving face is located opposite to the discharge pressure receiving face. The valve body moves along the axis thereof in accordance with difference between the pressures acting on the faces. A large difference between the pressures causes the highly pressurized refrigerant gas in the discharge chamber to leak into the suction pressure area through the clearance between the periphery of the valve body and the wall of the chamber accommodating the valve body. The gas leak deteriorates the refrigerant efficiency of the external refrigerant circuit.